Process of recovering carbon dioxide from gases



- two important discoveries.

r sass or ancovnamo o DIGIE M GASES I Frank Henderson May, 'lrona, ()alifl, assignor to erican Potash & Chemical Corporation, Trona, Califl, a corporation of Delaware No Drawing. Application September 30, 1962, Serial No. 460,278

4 Claims.

. gallon of absorbing medium used may be made large without necessitating the employment of high partial pressures of carbon dioxide in contact with the absorbing medium. By means of the present invention, I have been able to recover as high as 0.5 or more pounds of carbon dioxide per gallon of absorbing medium used, whereas with usual practice only about 0.1 pound of carbon dioxide per gallon of absorbing medium may be recovered.

A further object of the present invention is to provide a process of recovering carbon dioxide from gases, by which process I am enabled to reduce the amount of water vapor released from the absorbing medium during the steps of boiling oif the carbon dioxide therefrom. By means of the process of the present invention, I liberate only about one-fifth or less of the water vapor which is ordinarily liberated with carbon dioxide during the usual boiling ofi operations.

:The process of the present invention involves The first of these discoveries is that by maintaining in the absorbing medium a suiiic'ient concentration of potassium and borate material so that potassium 'pentaborate will be liberated during the absorption of carbon dioxide, the acidity of the absorbing medium may be maintained at a suiiiciently low value to permit absorption of greatly increased quantities of carbon dioxide without increasing the partial pressure of carbon dioxide over that occurring in usual-practice.

At the start of the absorbing operations, the borate material is mainly present in the tetraborate form, 1. e., with a ratio of 1320: to K of 2 to 1. Throughout this description, I refer to such tetraborate as the basic constituent of the absorbing liquor, as it has a great afllnity for the acidic gas, CO2. During the absorbing operations, the tetraborate material is converted into pentaborate in accordance with the following equation:

2KcBioO1a8HaO+ GKHCOs-l-HaO (Preclpitate) In this description, I refer to the pentabcrate (KzBmOm) as the "acid constituent; it has a ratio of B203 to &0 of 5 to 1, and is formed/in the process of my invention by virtue 'oithe added acidity provided by the absorbed carbon dioxide. According to the foregoing equation,

there is formed a s1udge" of carbonated liquor and precipitated potassium pentaborate octohydrate, and it is the precipitation ,of this acidic constituent from the solution present which enables my process to proceed with such a large absorption of carbon dioxide per gallon of absorbing medium.

The second basic discovery of the present invention is that the liberation of carbon dioxide from the absorbing medium may be greatly facilitated by retaining the precipitated potassium 'pentaborate in contact with the solution containing the absorbed carbon dioxide during the operations of boiling ofi the carbon dioxide therefrom.

boil off the absorbed carbon dioxide. During this operation, generally termed desorption" in the art, the precipitated potassium pentaborate octohydrate goes backinto solution, acidifying thesolution and thereby materially'aiding the liberation of the carbon dioxide. By the process of this invention, the carbon dioxide may be boiled off the absorbing medium while evaporating only one-fifth, or less, of the amount of water evaporated in usual practices.

The process of the present invention, together with additional discoveries and advantages of the invention, should be fully understoodfrom the following description of a number of examples of processes embodying the invention. T

Since the pr ocess is a cyclic process, the description of it may begin at any point in the cycle. I will describe the first example of a process embodying {the present invention, commencing the description with a statement of the composition of the sludge constitutingthe absorbing medium Iobt'ained at the completion of the absorbing operation. Such a sludge as obtained at 35 C. contained 27.6 gm. 0f potassium pentaborate octohydrate (KzBioOiliHzo) as a solid per gm. of excess water in the accompanying solution. 'l'he accompanying solution had the following composition;

, Grams Potassium tetraborate tetrahydrate I (KrBiO'zAHzO) 31.0

entaborate octo'nydrate Potassium p (KzBroOraBI-IzO) 4.3 Potassium bicarbonate (KHCO3)....-' 24.1 Excess water (H2O) 100.0

! Total solution 159.4

In accordance with the process of the present invention, the sludge oi precipitated potassium pentaborate octohydrate and liquor is heated to It should be understood that while the sludge assures oi the foregoing composition is well adapted for I use in the process of the present invention,

sludges of other concentrations 'or densities may be employed. The presence of sodium compounds in the sludge is not deleterious to the process, as more particularly pointed out in connection with the third example.

In theprocesa'such a sludge is heated to boil- 1 until a temperature of 100 C. is reached. During agitator, a thermometer reaching to the bottom of the flask, and a connection leading to an of!- side condenser, which condenser is strongly cooled. The lower end of the condenser dips into a smaller flask containing strong sulphuric acid, which is provided for receiving the c; ndensate and for scrubbing. the water vapor out of the evolved carbon dioxide gas. During the test, I place a sample of the sludge from the absorber in the agitated flask and applyheat both at the bottom (to cause gentle ebullitijon) and near the neck of the flask (to prevent condensation and reflux). By careful weighing of the two flasks, before and arter heating, the weight of carbon dioxide and water vapdr driven oil? from the absorber sludge may be determined.

this boiling oil process, most. if not all, of the potassium pentaborate octohydrate becomes dissolved in the solution by the time the solution 7 has been heated to about 65 C. Stated another Inthe processor my invention, the carbon dioxide is liberated very easily and with very littleconcomitantwater vapor at the start of the desorption (boiling bperations). As this proceeds, the temperature of. the solution rises and a greater proportion of the water vapor is expelled with the carbon dioxide gas. It is,

of course, within the province of the operator by reduces the amount of watervapor which it pletely reacted with the potassium bicarbonate (KHCO3) in accordance with the following equation: 2Kmwcmsmo+smco$+md (2m gm.) 14.1 i 0.4 gm.)

' =5K2B407.4H20+6C02 35.9 gm. (6.2 gm.)

In boiling oil carbon dioxide. from absorber honors, the cost of the product is largely determined by the quantity of water vapor that is evolved simultaneously with the carbon dioxide.

Since the evolution of the concomitant but useless water vapor requires heat, it is obvious that the less water vapor liberated during desorption, the less the cost of the procedure.' Such heat is generally supplied in the form of steam, and the demand for such steam in the desorption operation is generally spoken of as the steam consumption or the heat requirements of the process. In plant practice, the provision of economizers, heat exchangers, and counter-current towers affect the total quantity of steam required to some extent, but the amount of water vapor which is liberated with the carbon dioxide nevertheless largely determines the steam or heat requirements of the process. Accordingly, I find an important factor to be considered in evaluating such a-process-is to determine the ratio of water vapor to carbon dioxide liberated in the process. The ratio of water vapor to carbon dioxide which will be liberated in the process may be affected to some ex-.

tent by the equipment used, but to a considerable extent vis determined by the composition of the absorbing medium. In order to evaluate the composition of the absorbing medium in this respect, I, therefore, test the medium as follows:

In the test I employ a flask fitted with a sealed of my process to choose, depending upon various practical considerations, the temperature at which he desires to carry out the boiling ofl operations. This is illustrated by a series of tests I have made when boiling off an absorber sludge similar to that specified in Example 1.

' Ratio of water 00; (expel! Final boiling temp. at 0.) g

. expelled'ln S Y test weight 21 o. as o. 62 0. 81 0. 100 1.

1 All solids dissolved at as o.

I When, as in the first example of the process,- the boiling ofi operation is continued until the temperature of 100 C. is reached, there is secured one pound of carbon dioxide from the solution for each 1.2 pounds of water boiled off the solution. There is secured a yield of approximately 0.41 pound of carbon dioxide per gallon of sludge passed to the boiling oil operation. By boiling slightly harder (say to 102 C.) the yield oi carbon dioxide may be increased at the sacrifice of somewhat greater evolution of water from the solution. As much as 0.5 pound of carbon dioxide pergallon of sludge may be recovered. This represents a substantial advance over theusualpractice of the art in which only about 0.1 pound of carbon dixide is boiled oil a gallonoi starting material.

Moreover, the process of the present invention attains a high yield of carbon dioxide pergallon of material with the evolution of only a relatively small amount of water vapor (low steam or heat consumption). In the usual practice of the art, the steam consumption per pound of carbon dioxide liberated is 5 to 10 times that of the present example of the process of my invention.

After liberation'oithe carbon dioxide in the boiling ofl operation, all solids are usually in solution. The hot solution is then cooled and returned to the carbonating or absorbing operation. During this cooling operation, over-saturation is reached with respect to potassium tetraborate tetrahydrate (KHBQO'IAHEO), and such asoasec potassium tetraborate may be precipitated in some cases. I have found, however, that KzBsOmHzO has a strong tendency to resist crystallization, so that the cooling procedure a may not precipitateall, or even any,KaB4O1.4Hs0

ducted counter-current-the cold absorbing me'-- dium is introduced into the top of an absorption tower, while the gases are introduced into the bottom of the tower. During the absorbing operations, the potassium 'tetraborate' reacts with carbon dioxide, forming potassium pentaborate. The potassium pentaborate precipitates from solution as octohydrate (K2B1oO1a8I-I2O). Heat is liberated in these absorbing operations and may be continuously removed in the opera produced. When the absorbing operations are conducted counter-currently, the most denuded gases come into contact with the fresh absorbing .medium which is best adapted to remove the carbon dioxide therefrom. Where the fresh cold absorbing medium is supersaturated with KQBQO'IAHZO it has even a lower CO2 partial rpres- I sure than the absorption medium which has precipitated K2B407-4H20, and. is, therefore, more effective in absorbing carbon dioxide. Because of the high emciency of the absorbing liquor when supersaturated with KZB-iO'IAHZO to remove carbon dioxide, it may be fed into the tower at temperatures considerably higher than the temperature at which it is to be withdrawn from the tower. During the absorption or carbonation operation, the precipitation of the acid borate (KzBmOmBHaO) restrains the rise of acidity in the absorption medium and thereby permits a high absorption of carbon dioxide per gallon of absorption medium used.

While I have referred to the acid borate precipitated during the carbonation operation as potassium pentaborate octohydrate (KzBmOmBHaO) I have occasionally encountered the formation 7 and precipitation ofnothe r acid borates (more acid than X23101), about whose composition I therefore, included herein as within the scope or my invention.

In the second example of the process of my invention, I maintain a greater content of potassium borate material in the medium than described in connection with the first example. By so-doing, I may obtain either a greater yield of carbon dioxide per gallon of the medium employed, or I may obtain the same yield with a low steam or heat consumption, or both. These advantages are attainable only by handling sludges of greater density.

In the second example of'the process, I start the process .by adding 92A partsby weight or K2B4O'L4H2O to 99.3 parts by weight oi water. The resulting medium is then employed to absorb carbon dioxide from gases containing the same until all the solid KzBeOvAl-IsO has been dissolved .at 35 C., for example, and the solution hasabsorbed 10.6 parts by weight of carbon dioxide, i. e., contains 24.1 parts of "KHCO: (CO2 expressed as KHCOa). By so carbonatmg, a precipitate of 42.9 parts by weight of KaBmOmiZHzO will be formed. This 42.9 parts by weight of K2B1oO1e.8H2O may be compared with the 27.6 parts precipitated in the first example. The solution formed has exactly the same composition at 35 C. as the solution described-in the first example. The fact that a greater quantity of acid borate is precipitated and is present in the sludge does not alter the partial pressure of carbon dioxide over the sludge; such partial pressure beingthe same as in the first examplenamely, 64 mm. at C. or 77 mm. at C. This larger amount of acid borate in. the sludge of the second example provides an even greater reservoir of,

acidity for use during the boiling off or desorption operations, with the result that either (1) the desorption may be carried out more easily and with less consumption of steam when the same yield (as in Example 1, i. e., 6.2parts per 100 parts excess H20) of carbon dioxide is desirm; or (2) a greater yield of 002 per gallon of sludge may be obtained while suflering the same steam consumption as in the former case.

If such greater yield is desired, it is obvious, of course, that the KHCO: content of the desorbed solution will have to be reduced to a lower value than in the former case-'-say to 5 parts of KHCO: per 100 parts of excess H20. j

l The following tabulation of "boiling off" data, similar to that explained in connection with the first example, shows the advantages just enumerated for Example 2:

Percent of Final boiling temp. (3'.

1 All solids lied dissolved at this temperature.

On a comparable basis parts excess water in the absorber sludge), the boiling o8 operations liberated, 27 more CO2 in Example 2 than in Example 1, and did so with a lesser evolution of water vapor (per pound of liberated C02). The indicated one .pound of carbon dioxide per pound of water vapor duringv desorption represents an extremely economical operation-a great advance over past practice.

Ratio of water to C n In the second example of the process, the deaccuse sorbed medium is cooled and returned to the car-' bonating operation as in Example 1, thereby'es- KQB40'L4H20.

r an o esmo. The solution composition at 35 KaBmOraSI-IzO 7.9 KHCO: 24.0

Na2B4O-zJOHzO 32.5 Excess H20 100.0

borate with the potassium. bicarbonate in the se lution, as shown by the following equation:

2KzBioO1c+6KHC0s-l-heat=5KzB4O7+6C0z While I have described in both of the foregoing examples that the carbonating operations are conducted to produce a solution containing about 24 parts of KHCO: in the absorption solution per 100 parts of excess water, the carbonation could easily be carried to a lesser degreesay 18 to 20 parts, or the carbonation could go as high as saturation with KHCOz-but in such case, only by operating with-increased carbon dioxide partial pressures-that is, 96 mm. at 25 C., 148 mm. at C., and 234-mm. at C.

Under certain economic considerations, such in-- creased partial pressures would not be prohibi-.

pressure.

In the foregoing two examples of my invention,

the absorption medium has included only the potassium borate and carbonate compounds.

However, the presence in the absorption medium of sodium compounds is not deleterious. Potassium tetraborate is not at present a cheap material of commerce. Potassium pentaborate is, on the other hand, a cheaper product than KaBrO-z duetothe ease of efficiency with which it may be prepared. By substituting considerable KzBmOreSHrO and a suitable alkaline compound, v

such as NazCOs, for the relatively expensive KzBrOvAHaO. a very useful absorption medium may be produced. In fact, this absorption medium has certain advantages over the absorption tribute various advantages.

To start the process, I produce an absorbing medium by adding to 110.6 parts by weight of water the following reagents: I

Parts Kano-(Arno a I 71,4 K2B1oO1c.8H2O 37.8 NMCO: 9.0

This'absorption medium is then carbonated until all of the solid K2B4O'L4H2O dissolves (requiring 6.7 parts CO2 for this first preparation step). At this stage, the absorption medium contains the same amount (per 100 parts excess water) of solid KaBroOrciiHzO as the second example set forth above-namely. 42.9 parts of solid Percent of CO: (expelled g g Final boiling temp, C. at 100 C.) er] 9 expelled in evolved (by test ' All solids in solution at 71 C.

In boiling ofl carbon dioxide in Example 3 all solids go into solution at about 71 C. If the boiling oil operations are continued until a temperature of about 100 C. is reached, the actual quantity of carbon dioxide expelled (per 100 parts of excess water in the absorber sludge) is about the same as in the case of Example -2. At the completion of the desorbing operations, all solids are in solution. The solution may then be cooled and recycled to the absorption or carbons-ting operations;

The cooled solution exceeds saturation at 33 C. with respect to potassium tetraborate tetra hydrate, but such material may be retained in solution in a supersaturated state. When this is done, I find that the absorbing medium when cooled to 33 0. showed a very low partial pressure of carbon dioxide-namely, 17 mm.

In the foregoing examples of my invention, I have described operations in which the carbonation is conducted so that the final carbonated sludge will be just saturated with the tetraborate in question, but various modifications ar possible in practice. Less K2B4O'IAH2O may be added to the cycles liquors during make-upor the carbonation may be carried further with respect process of the present invention, so long as sufficient reagent is added to insure the absorption of carbon dioxide and the precipitation of an acid potassium borate. such as K2B1oO1s.8HzO.- Furthermore, in the process of this invention, an even greater quantity-of the reagent K2B40'L4H2O may be used than that necessary to produce the solution, which after carbonation is just saturated with K2B401.4HzOallowing some solid One advantage of such an excess is that it eliminates the necessity for exact control of the extent of the carbonation operation.

I have mentioned the tendency of the desorber liquor to become supersaturat i L cooling with respect to the alkaline borates and Na2B4Oml0H2O) and have indicated that such a phenomenon is not disadvantageous. I have also found that the acidic borate (KeBroOrcBHzO) may be sluggish in precipitating during carbonation. This is definitely undesirable and the operator should take steps to avoid the same. This can be accomplished by good agitation during can bonation, or by seeding with KzBmOmSHzQ, or by both methods. Usually, in a continuous process, once established, little trouble is encountered on this score.

Reference is hereby made to my copending applications Serial Number 507,298, filed October 22, 1943, and Serial Number 519,340, filed January 22, 19 44, which claim related subject matter.

I claim:

1. A process of extracting carbon dioxide from gases containing the same, which process comprises absorbing carbon dioxide from such gases to form potassium bicarbonate by reaction with an absorbing medium containing such concentrations of potassium borates that pentaborate is formed by said reaction and potassium pentaborate octohydrate is precipitated to form a sludge, then heating the resulting sludge to such an extent that the potassium pentaborate octohydrate reacts with the dissolved P tassium bicarbonate to form and expel carbon dioxide and to form tetraborate in solution, and cooling and returning the resultant absorbing medium to the first operation.

2. A process of extracting carbon dioxide from sorbing carbon dioxide from such gases to form potassium bicarbonate by reaction with an absorbing medium containing such concentrations of potassium borates that pentaborate is formed and potassium pentaborate octohydrate is precipitated by such reaction, said absorbing medium being initially metastably supersaturated with potassium tetraborate tetrahydrate, then heating the resultant sludge to such an extent that the potassium pentahorate ectchydrate reacts with dissol ed potassium bicarbonate to form and expel carbon dioxide and to form tetra-' borate in solution, and then cooling the resultant absorbing medium and returning the same to the speration. 3. A process of extracting carbon dioxide from gases containing the same, which comprises ab= sorbing carbon dioxide from such gases to form potassium bicarbonate by reaction with an absorbing medium containing such concentrations of sodium and potassium borates that pentaborate is formed and potassium pentaborate octoie hydrate is precipitated by the reaction, then heat.-

ing the resulting sludge to such an extent that the potassium pentaborate octohydrate reacts with. the dissolved potassium bicarbonate to form .and expel the absorbed carbon dioxide and to form tetraborate in solution, and cooling and returning the absorbing medium to the first-mentioned operation.

i. A process of extracting carbon dioxide from gases containing the same, which process com- 25 prises absorbing carbon dioxide from such gases to form potassium bicarbonate by reaction with an absorbing medium containin such concentrations of potassium borate that pentaborate is formed and potassium pentaborate octohydrate is precipitated by such reaction, the absorbing medium being initially metastably supersaturated with potassium tetraborate tetrahydrate, the reaction being continued until the resulting sludge of potassium pentaborate octohydrate solution is near saturation but not metastably supersaturatgases containing the same, which comprises abi ed with respect to potassium tetraborate tetrahydrate, heating the resulting sludge to such an extent that the pentaborate reacts wtlh the dissolved potassium bicarbonate to form and expel so the absorbed carbon dioxide and to form tetraborate in solution, and finally cooling and returning the absorbing medium to the first operation.

FRANK HENDERSON MAY. 

